The aim of this study is to determine the accuracy of VEC MRI measurement of the Qp/Qs ratio in children and adults with atrial and ventricular septal defects using the Fick method as the reference standard. Background: Atrial and ventricular septal defects are the most common forms of congenital heart disease. Typically, in these conditions, blood flows through the defect from the systemic to pulmonary side of the circulation. This excess blood flow to the lungs or shunt may result in congestive heart failure, poor growth, and pulmonary hypertension. The magnitude of the shunt is numerically expressed as the ratio of pulmonary to systemic blood flow (Qp/Qs ratio). Assessment of the Qp/Qs ratio is often a key component in the clinical management of atrial and ventricular septal defects including decisions regarding interventions for closure. The current clinical reference standard for measuring the Qp/Qs ratio is based on the Fick principle and requires cardiac catheterization and measurement of blood oxygen saturation in multiple cardiac chambers. As this procedure is invasive, necessitates sedation,and involves radiation and contrast agent exposure, an alternative technique would be valuabe to both the clinician and patient. The most commonly used non-invasive methods to estimate the Qp/Qs ratio are Doppler echocardiography and radionuclide scintigraphy. Unfortunately, the utility of these approaches is often limited by suboptimal accuracy, poor acoustic windows, or insufficient anatomic resolution. Velocity-encoded cine MRI (VEC MRI) is an emerging magnetic resonance imaging technique for measuring regional blood flow. The phase-velocity method is based on the principle that hydrogen nuclei flowing through a magnetic field gradient accumulate a predictable phase shift in spin that is proportional to their velocity. To determine flow volume through a vessel lumen, phase map images of the vessel cross-section are constructed for multiple instants during the cardiac cycle. The intensity of each pixel within the lumen corresponds to the velocity of blood flow at that location. The mean velocity within the lumen is calculated and multiplied by the cross-sectional area to equal the blood flow rate at that instant of the cardiac cycle. Integration over time of the instantaneous flow rates throughout the cardiac cycle yields the net blood flow volume per cardiac cycle. Thus the Qp/Qs ratio can be measured by acquiring VEC MRI images of the cross-section of the pulmonary artery and ascending aorta. The investigators and others have successfully validated the accuracy of VEC MRI measurements in vitro.